Radar system and method for determining the point of burst of a projectile



May 4, 1965 Filed April 1, 1963 W. C. BROWN RADAR SYSTEM AND METHOD FORDETERMINING THE POINT OF BURST OF A PROJECTILE l2 Sheets-Sheet 1 May 4,1965 w. c. BROWN 3,182,318

RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OF APROJECTILE Filed April 1, 1963 12 Sheets-$heet 2 May 4, 1965 w. c. BROWN3,18 ,318 RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OFA PROJECTILE Filed April 1, 1963 12 Sheets-Sheet 3 y 4, 3965 w. c. BROWN3,182,318

RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OF APROJECTILE 12 Sheets-Sheet 4 Filed April 1, 1965 May 4,, 1965 w. c.BROWN 3,182,318

RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OF APROJECTILE Filed April 1, 1963 12 Sheets-Sheet 5 AM/L/ P/W/ :D f/f 1 C)CD #5 #w -7 1 19 7a. [A m ,5

5 (5 4/1/19 [4 ,P/W/ #5 w 11 C) 7 CD 9v 4/141 7M %,;;H}ZIP v [6 PM 427#5 h C) C) iv M Y 2% a May 4, 1965 w. 0. BROWN 3,182,318

I RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OF APROJEGTILE Filed April 1, 1963 12 Sheets-Sheet 6 I "ZQQZP AMH 0144/ AM/e H5 w 1 (3 Q? Z z 7e- M M5 PM (E May 4, 1965 w. c. BROWN RADAR SYSTEMAND METHOD FOR DETERMINING THE POINT OF BURST'O F A PROJECTILE l2Sheets-Sheet 7 Filed April 1, 1963 @Q WTY I W i y $&& w 1 I 1 1 y 1 Q 4H P e M 1/ 4 AL n N w i W i 2 |1 l/Llll l b L 4 D J a; m a r M a m @4 5r 7 MI, W, 7 H W 0, mu N WW7 ffl/wllll 6 W 4 ga w y n 7 ll| V an I I I II WW TM Wm M LG 7 A May 1955 w. c. BROWN 3,182,318 RADAR SYSTEM ANDMETHOD FOR DETERMINING "THE POINT OF BURST OF A PROJEGTILE Filed April1, 1963 12 Sheets-Sheet 8 May 4, 1965 w c BROWN 3,182,318

RADAR SYSTEM AbiD METHOD FOR DETERMINING THE POINT OF BURST OF APROJECTILE Filed April 1, 1963 12 Sheets-Sheet 9 M y 4 1965 w c BROWN3,182,318

RADAR SYSTEM AIiD METHOD FOR DETERMINING THE POINT OF'BURST OF APROJECTILE Filed April 1, 1963 12 Sheets-Sheet l0 .FW 5 7 7J Z1 5 54 lu/P// 5/48 M/? I May 4, 1965 w. c. BROWN 3,182,318

RADAR SYSTEM AND METHOD FOR DETERMINING THE POINT OF BURST OF APROJECTILE Filed April 1, 1965 12 Sheets-Sheet l1 Fry-J1.

W. C. BROWN RADAR SYSTEM THE POINT Filed April 1, 1963 May 4, 1965 ANDMETHOD FOR DETERMINING OF BURST OF A PROJECTILE 12 Sheets-Sheet 12United States Patent 3,182,318 RADAR SYSTEM AND METHGD FOR DETERMHN- INGTHE POINT OF BURST OF A PROEECTILE William C. Brown, Gttawa, Ontario,Canada, assignor to National Research Council, Ottawa, Ontario, Canada,a body corporate of Canada Filed Apr. 1, 1963, Ser'. No. 269,285 Claimspriority, application Canada, Jan. 23, 1963, 867,187 Claims. (Cl. 343-7)This invention relates to a radar system and method, for use inobtaining echoes from projectiles, for the purpose of determiningaccurately the trajectory of such projectiles and the location of pointsthereon.

Although the invention may, and generally will, be incorporated in aradar system designed primarily for 10- cating enemy Weapons byobtaining echoes from projectiles fired by such weapons, the presentinvention is itself concerned solely with the radar observance ofprojectiles fired by friendly guns.

Before being able accurately to hit a target at a desig nated mapreference it is necessary for gunners to follow a procedure known asregistration of fire. This procedure constitutes the calibration of eachweapon to take into account various factors such as barrel wear andother aging of the weapon parts, as well as meteorological conditions.Effective registration of fire in the traditional manner involves theneed for two or more forward observation posts. Trial rounds are fired;the fall of shot is observed from the observation posts; and fixes areobtained on such positions. This information is then trans mitted to theguns for the necessary corrections to be made. Many rounds and valuabletime are lost in this way.

The present invention is concerned with a radar system which greatlyaccelerates this procedure as well as providing accurate information andavoiding the need for forward observation posts. The invention providesmeans for obtaining an accurate fix on the point of burst of aprojectile which has been fused for bursting within observable range ofa telescope mounted on the radar system. This is normally best achievedby fusing the projectile to burst in the air above the ground. Such airbursts can be comparatively readily observed from a substantialdistance, without impediment by ground objects or variations in groundlevel. However, it will be assumed in the description which follows thatfusing for air burst is the method adopted, since it is of universalapplicability.

United States patent application of William C. Brown et a1. Serial No.269,367 filed April 1, 1963 describes a basic radar system for theobservance of enemy projectiles. United States patent application ofCharles R. Clemence et al. Serial No. 269,284 filed April 1, 1963describes a refinement of this system in which a similar radar system isemployed both to watch enemy projectiles and to observe friendlyprojectiles aimed at the enemy weapon. In this way it is possible todetermine where the friendly projectiles are falling in relation to theposition of the enemy weapon as calculated from observance of thetrajectories of its own projectiles. It will be appreciated that theenemy weapon itself will normally be hidden from direct visual or radarobservation.

The present invention is concerned with those aspects of a radar systemprovided for watching solely friendly projectiles, for the purpose ofregistering the friendly guns. Nevertheless, for the purposes of thedescription which follows, it has been assumed that the features of thepresent invention are incorporated into a radar system similar to thatdescribed in such prior application No. 269,367. Such a system providesradar means for determining at least two points (intercepts) throughwhich 3,182,3i8 Patented May 4, 1965 a projectile passes, and a computerfor calculating from such intercepts the point of intersection of thetrajectory of the projectile with a working plane. The working plane isdefined as the one including the line between the radar system and thetarget point and all horizontal lines perpendicular to said line. Thetarget point is thus the point of intersection of the projectiletrajectory with the working plane. When the radar system is concernedwith 10- catiug an enemy weapon on the ground or with locating theposition of the fall of friendly shot on the ground, the angle of theworking plane will be chosen to give a ground location for the targetpoint. When the target point is above the ground, as is the case whenlocating the position of an air burst, the working plane iscorrespondingly modified to pass through both the radar system and suchelevated target point. When desired for greater accuracy, the timeinterval required for passage of the projectile between a selected twointercepts is also inserted into the computer.

To this end, the radar system of the invention comprises (a) Means foremitting two closely vertically superposed, mutually divergent,generally horizontal, effectively continuous, upper and lower radarbeams,

(b) Means for displaying echoes returned by a projectile travelling in atrajectory intersecting said upper and lower beams and for determiningrange and azimuth values of such intersections measured from the radarsystem in relation to a known azimuth datum,

(c) A computer,

(d) Means for supplying the computer with said determined range andazimuth values, with the value of the angle (0) the lower beam makeswith the horizontal, with the value of the angle (at) between beams andwith the value of the angle a working plane makes with the horizontal,

(e) Said computer including means for calculating at least approximatelycoordinates of a target point on said working plane through which suchtrajectory extends,

(1) And means for observing the point of burst of a projectile and forsetting the computer for a working plane passing through such observedpoint of burst.

The invention also has a method aspect which can be defined in its broadscope as (a) Emitting from a radar system two closely verticallysuperposed, mutually divergent, generally horizontal, effectivelycontinuous, upper and lower radar beams to intersect the trajectory ofsaid projectile, the lower beam lying above said point of burst,

([2) Determining the angle (0) the lower beam makes with the horizontal,and the angle (a) between the upper and lower beams,

(c) Observing the point of burst and determining the angle (A9) that aworking plane extending from the radar system through said point ofburst makes with the lower beam,

(d) Determining the angle 5) made with the horizontal by the workingplane from the equation =0A6,

(6) Displaying on a range-azimuth radar display echoes returned by saidprojectile during intersection of said upper and lower beams,

(1) And employing the differences in range and azimuth of theintersections so displayed, together with the values of said angles, todetermine coordinates of the point of burst.

Details of the extrapolation equations employed and their manner ofsolution appear from the specific description that follows which,together with the accompanying drawings, illustrates two manners inwhich the invention may be carried into practice. It is to be understoodthat the radar system specifically illustrated in the drawings and themethod of operation described in relation thereto is furnished by way ofexample of the invention only, the broad scope of the invention beinglimited only by the appended claims.

In the drawings:

FIGURE 1 is a general perspective view of a radar system according tothe invention in operation,

FIGURE 2 is a perspective view on a larger scale of the antennaassembly,

FIGURE 3 is a rear view of the antenna assembly of FIGURE 2,

FIGURE 4 is a fragmentary diagram showing the connection of a telescopeon the antenna assembly to the computer,

FIGURE 5 is a diagram of a typical trajectory of a projectile fused forair burst.

FIGURE 6a is a plan view of the area scanned by the radar system,

FIGURE 6b demonstrates the manner of presenting such area of scan(FIGURE 6a) on a radar B-scope during a long range searching sweep,

FIGURE 60 shows a portion of the presentation of FIGURE 6b enlarged asit appears for a short range sweep,

FIGURE 7a is a simplified front view of a portion of a radarcontrolpanel illustrating diagrammatically the appearance of an echo ofa projectile on the screen,

FIGURE 7b is another view similar to FIGURE 7a, a short time later inoperation,

FIGURE 7c is yet another view similar to FIGURES 7a and 7b at a laterstage,

"FIGURE 7d is another similar view at yet a later stage in the radarobservance of the projectile,

FIGURE 7c is a view similar to FIGURES 7a to d showing the marks made bythe operator which remain after the projectile echoes have faded, and amanner of use of a marker spot.

FIGURE 7 f is a view similar to FIGURE 7e at a later stage in operation,

FIGURE 8 is a general overall circuit for the radar system,

FIGURE 9 is a more detailed illustration of the portion of the circuitof FIGURE 8 principally concerned with calculating the range of thetarget point.

FIGURE 10 is a more detailed illustration of the portion of the circuitof FIGURE 8 principally concerned with calculating the elevation of thetarget point and providing certain parameters to the range and azimuth vcircuit portions,

Overall system (FIGURE 1) FIGURE 1 shows the radar system RD mounted ona vehicle V being used to observe the trajectory T of a projectile firedby a friendly gun G. The area towards which the gun is firing is behinda row of hills H, so that the ground level at such location is notvisible. The

projectile will be fused for air burst at a certain height above theground, which will be position AB. It is the coordinates of this pointAB that the gunners wish to know toadjust their guns. The antennaassembly of the "radar system RD. provides a narrow beam substantiallycircular in cross-section having a width of approximately 16 mils inboth directions. The system causes this narrow beam to scan horizontallythrough approximately 400 mils angle by approximately 40 mils (2.25") atbeam centers. This action defines, by the narrow beam locus, twovertically superposed fan-shaped beams, each scanned 20 times persecond, hereinafter referred to as the upper and lower beams. Thiseffect is achieved by use of a Foster type scanner SC similar to thatdisclosed in Foster US. Patent No. 2,832,936 issued April 29th, 1958,and modified to provide a dual beam in a manner similar to thatdescirbed in Mobile Radar Finpoints Enemy Mortar Positions; by M. S.Iaifee et a1. Electronics Sept. 18, 1959, and, page 34 et seq. Thescanner SC is placed at the focus of a semi-parabolic cylindricalreflector RF which produces the two focused beams.

Antenna construction (FIGURES 2 t0 4) As best seen from FIGURES 2 and 3,the scanner SC and the reflector RF are mounted on the roof of thevehicle V by means of a gimbal-like mounting consisting of a first frame11 arranged to rotate about an axis-defined by pins 12, and a platform29 arranged to rotate relative to the frame 11 about pins 21 defining anaxis perpendicular to that of the pin 12. Mercury switches MSA and MSEare employed to control respectively an actuator A for tilting the frame11 relative to the vehicle roof, and an actuator B for rotating theplatform 20 relative to the frame 11. These mercury switches maintainthe platform 20 horizontal at all times regardless of the attitude ofthe vehicle V by means of suitable control circuits. These circuits formno part of the present invention and will not be further discussed.Details of the construction of the antenna assembly and an example ofcontrol circuits for the automatic levelling thereof are contained inClemence et al. United States patent application Serial No.'269,363filed April 1, 1963.

The platform 20 carries a turntable 26 which can be rotated on theplatform 29 about a central vertical axis by a motor (not shown). Inthis way the antenna array is rotated in azimuth to provide completeradar coverage through 6,400 mils (360).

Mounted on the turntable26 is a transverse girder 30 formed at each endwith an upstanding bearing 31 carrying a pin 32. The pins 32 define ahorizontal axis about which antenna frame members 33 rotate under thecontrol of a third actuator C. Frame members 33 carry the scanner SC aswell as the reflector RF. The reflector RF is capable of being loweredto a stowage position by rotation about further pins 51, but this is amovement with which the present specification is not concerned. Mountedbeneath the bottom edge of the reflector RF so as to look along thecenter line of the lower beam is a telescope TE. When actuator C isextended or retracted to vary the angle of sight of a whole antennaarray, the angle of sight of the lower beam and of the telescope remainequal to each other at all times. This angle related to the horizontalis designated 0 in FIGURE 5. The angle,

dimensions. Line 42 represents a movable graticule under the control ofknob 43. The point AB marks the point of air burst ofa projectile, withwhich point the graticule 42 has been aligned by an operator fora'purpose which will become clear from the description below. Knob 43 isconnected by a shaft H43 to a synchro transmitter '44 V for transmittingthe position of knob 43 and hence of graticule 42 electrically to asynchroreceiver 45 and hence via shaftHdS to a repeater in the form ofan indicating pointer 14 situated at the control system inside thevehicle V.

panel of V the Mathematics of the computations to be made (FIGURE 5)Before considering the detailed nature of the display which appears onthe radar screen as a result of a projectile passing through the upperand lower beams, it is necessary to consider the mathematics of theproblem, taking as a first assumption that the echoes received from eachbeam can be resolved into a single point on the trajectory T of theprojectile, the range and azimuth of which point (intercept) thus becomeknown. The angle of sight of the lower beam to the horizontal is knownby the setting applied to the antenna array over which the operator hascontrol.

FIGURE shows two intercept points a and b on trajectory T determined bythe radar system RD at ranges RI and R2 respectively. The angle 4:represents the difference between the horizontal plane HP through theradar system RD, and the working plane WP which is the plane in whichboth the radar system RD and the target point lie. When the radar systemis being used to determine the position of an enemy mortar M, theworking plane is set to pass from the radar system RD through such pointM. When the system is being used to detect the position of an air burstAB, the working plane must be effectively moved up to a new position WPsubteuding a new angle 5' with the horizontal plane HP at the radar RD.The working plane, by definition, always passes through the targetpoint.

It can be shown that the true target position in polar coordinates fromthe radar system RD, where Rm is the target range and Am is the anglebetween North and the line of sight of the target are given by equationsRm=R1+K'[AR+a(0+%)Rm 1) and Am =Al+KAA (2) where K=KQ and

Where AR=Rl-R2, and AA=A l-AZ, Al and A2 being the azimuth anglesrespectively of points a and b. AT is the time of flight of theprojectile between intercept points a and b, and g is the gravitationalconstant.

The manner of derivation of these equations is explained in full in theabove mentioned prior United States patent application Serial No.269,367, and it is believed unnecessary to repeat their derivation inthis specification.

A computer is provided to solve these equations, as is explained below.The target position in polar coordinates Rm and Am can be converted torectangular (Cartesian) coordinates by means of a resolver the output ofwhich is Rm sin Am and Rm cos Am.

Besides determining the position of the target point in plan, thecomputer will provide an elevation of this position. The elevation ofthe target position relative to the radar is Rm sin (15. For simplicity,and because t is always small, this can be taken as Rm.

Nature of echo presentation and operators procedure (FIGURES 6 and 7)Attention is now directed to FIGURES 6a, b and c. FIGURE 6a shows a planview of a typical sector SR scanned by the radar RD. Two composite echodisplays E and E produced by the lower and upper beams, re-

6 spectively, are shown. FIGURE 6b demonstrates the manner in which thesector SR is presented on the screen S of a B-scope, that is to say ascope which exhibits azimuth along the horizontal am's and range alongthe vertical axis. After having detected a weapon firing, the operatorwill enlarge the critical area of the sweep as demonstrated by FIGURE6c.

The echo displays actually received in practice are more complex thanthose illustrated in FIGURES 6a, b and c,. and attention is nowtransferred to FIGURES 7a to f for a more detailed discussion of thenature of the display on the screen actually observed by the operator.

As a projectile enters the filed of scan of the lower beam, an echo E1is displayed on the screen S by a group of individual signal returnsresulting from a single passage of the narrow beam across theprojectile. The center of the leading (lower) edge of this echo (pointCl) represents the true position of the object (projectile) beingobserved. As the beam continues to sweep, a series of such echoesappears on the screen S. These echoes are indicated as E1 to E5 inFIGURE 7b and make up the composite echo E of FIGURES 6a to c. Inreality there may be many more than five individual echoes in thisseries. Echoes E1 to Ed are shown in broken lines because some fadingwill have taken place by the time the last echo E5 appears. The duty ofthe operator is to observe or mark the center points CI and C5 of theleading edges of the first and last echoes and to estimate the meanspoint CML between these two extreme center points. The screen S isprovided with .an outer surface that can readily be marked by theoperator using a suitable stylus. He may mark points C1 and C5 on thescreen as echoes El and E5 appear, and then estimate and mark the meanpoint CML, but an experienced operator will be able to estimate quiteclosely the mean point CML and mark it on the screen merely byobservation of the series of echoes, without finding it necessary to gothrough the preliminary stage of actually marking points CI and C5. Thevital point to obtain as accurately as possible for the purposes ofsubsequent operation of the computer is the mean point CML of thecentres of he leading edges of the series of echoes LR resulting fromthe lower beam LB.

Assuming that the mortar is firing from left to right and towards theradar system RD, the second series of echoes UR detected by the upperbeam UB and shown as composite echo E in FIGURES 6a to 0 appears firstas an echo E5 (FIGURE 7c) and continues down to echo E10 (FIGURE 7d).These echoes of the second series will similarly have leading edgecenter points C6 to C10, the mean point of which is designated CMU. Theupper beam echoes will appear in a lower position on the screen S thanthe lower beam echoes when the mortar is firing towards the radar, sincethe range will have shortened somewhat by the time the projectilereaches the upper beam.

Thus, by the time all the echoes have faded from the screen S, theoperator will have marked at least the points CML and CMU, which pointsrepresent the mean positions of the projectile as it passed through thelower and upper beams respectively. It has been found in prac tice thata skilled operator can assess the positions CML and CMU to an acceptabledegree of accuracy even though estimating these points requires visualand manual dexterity.

As well as carrying out the functions just described, the operator willtime the interval the projectile takes to pass from the lower to theupper beam. He can do this either by comparing the first echo of eachbeam, the last echo of each beam or any pair of corresponding points onthe two beams. It has been assumed in FIGURES 7 that he uses the firstmethod and records the time between the respective first echoes El andE6 of the lower and upper echo series LR and UR. For this purpose, theoperator will push the switch HS with his left hand,

' function.

to leave his right hand free for marking the screen S.

'A second, parallel operating, hand switch HS is provided forleft-handed operators. As FIGURE 7a indicates by an arrow, the operatorwill push in the switch HS immediately on appearances of the first echoE1 of the lower beam. This operation will start the timer TM. When thefirst echo E6 of the upper beam appears (FIG- URE 7c) the operator willagain push the hand switch 'HSto stop timer TM which will now remain inits new position indicating AT, the time of travel from the lower to theupper beam by the projectile.

As FIGURE 72 demonstrates, the operator is left, after passage of aprojectile, with two points CML and CMU marked on his screen S, and ATrecorded in the timer TM.

The screen S is also provided with a marker spot MS which is anelectronic marker produced by conventional circuitry in the radartransmitter-receiver combination and synchronized with the scope sweepso as to occupy a single desired position on the screen S determinedhorizontally by an azimuth marker handwheel AMH and vertically by arange marker handwheel RMH. Reference may be made to E. F. V. Robinson,Canadian Patent No. 580,247, issued July 28, 1959, for a description ofa system for achieving this result. The operator first moves the markerspot MS by means of the handwheels'AMH and RMH to coincide with thepoint CML and when he has achieved this coincidence he presses a footswitch FS (FIGURE 7e). Depression of switch FS brings the computer intofull operation, as will appear in more detail below. Althoughillustrated simply, the switch FS is a toggle switch of the typecommonly employed to raise and lower the headlights of an automobile,that is until reactivated by a further depression of the operators footto be reversed. As demonstrated by FIGURE 77, after closing switch PSthe operator moves the market spot MS with handwheels AMH and RMH to thepoint CMU, while switch FS remains closed. In this way, the

operator feeds into the computer the difference in range 'AR'and thedifference in azimuth AA between these two points.

Operation of the computer (FIGURES 8 to) For an understanding of themanner in which the target 3 point is calcuated from the informationavailable, reference will now be made'to FIGURES 8 to 12.

described. a

The computer portion of the circuitry which the remainder of FIGURE 8illustrates in general layout can conveniently be roughly divided intoportions which deal respectively with range, elevation, azimuth andinformation display. These circuit'por-tions are treated separately andin more detail in FIGURES 9, 10, 11 and 12 respectively. jAlthough thesecircuits are interrelated sufliciently to require somereference to eachother in understanding, the description which follows will, as far aspossible, take each circuit separately and examine its composition and(In all these circuit diagrams, broken lines signifymechanicalconnection, full lines electrical connection.)

The range circuit portion (FIGURE 9) The range marker handwheel RMHpreviously describedin connection with FIGURE 7 controls movement of themarker spot MS on the screen S in the range to say a switch whichremains in each acquired position 7 its rotation is transmitted by shaftH1. Goniometer assembly G1 controls the range position of the markerspot MS on the screen. It also, through shaft H4, operates a cam CAM7which controls a pulse repetition frequency switch PRFS. This switchchanges over the radar system from short to long range.

At the same time the handwheel motion is transmitted by shaft H1 to afirst input of a mechanical differential D1. Thus, when the marker spotMS is moved to the point CML in FIGURE 7a, the range R1 of such point isfed into the differential D1. As already explained,

once the marker spot MS has been aligned with point.

CML, the operator operates foot switch FS which remains closed. As shownin FIGURE 9, foot switch FS serves to energize a mechanical clutch GL2which now connects the shaft H 1 of the handwheel RMH to shaft H3 whichforms another input to differential D1 acting in opposition to shaft H1.As a result, when the marker spot MS is moved by the operator towardspoint'CMU (FIGURE 7 the output of differential D1, shaft H2, remainsstationary, since AR (the range difference between points CML and CMU)is being inserted twice in opposite senses into differential D1. Theposition of shaft H2 represents the range value R1, while shaft H3 ismoved a distance equal to- AR.

Foot switch FS energizes relay RY, one pair of contacts RYI of which isopened to de-energize a clutch CLI which had hitherto been holding shaftH3 under the control of a motor M1. Contacts RYZ which are closed byenergization of the relay RY serve to ground the input to a motor returnamplifier MR1 de-energizing motor M1. The zeroing function of this motorM 1 will be described below in connection with the resetting of thesystem.

The factor -AR is transmitted by shaft H3 to a movable slider on aresistor RR2. This part of the circuit is concerned with simulatingEquation 1 above, which for convenience is here repeated.

Rm=Rl+K['AR+a 6+; Rm] 1 If the last term is called I, this equationbecomes Rm=Rl+K'(AR+J) (1 And if numerical values are inserted, thisequation becomes The factor I is added to AR by means of resistors RRSand RR9 arranged in series with resistor RR2. Resistors RR8 and RR9 eachhave a movable slider controlled through a shaft H6 by a motor M2. whichis made to turn by an amount representing the factor I, as will now beexplained. By definition I is equal to the angle of sight'fi plus aconstant, multiplied by the'range Rm, all divided by a further constant.The range Rm is inserted by a shaft H9 controlling a slider ona resistor'RRS, and 0 product of the positions of the'sliders on resistors RRS andRR6, namely Rm(6+a constant). Servo-amplifier SAI energizes motor M2 todrive'its shaft Ha to reorient the position of a grounded slider on aresistor RR7-in such a way as to restore the input to the amplifier SA);to zero.

This servo-loopthus maintains the shaft H6 at all times a at a positionrepresentativeof the function J, the constant 9 in the denominator ofthis expression being provided by the mechanical ratio between shafts H6and HM.

As noted above, shaft H6 is connected to the movable sliders onresistors RRS and RR9 whereby the combination of these resistors withresistor RRZ generates the function AR+].

It is now necessary to form the product of such latter function and thefunction K. The function K is inserted into the system of FIGURE 9 atthe slider of a resistor RR3 by a shaft H7 the position of which iscontrolled in the manner subsequently described in connection withFIGURE 10. Resistor RR3 is series connected with the slider on resistorRRZ by contacts KY3 which are closed when relay RY is closed and theresulting product output is connected through closed relay contacts RY4to a servoamplifier SAZ which controls a motor M3 the shaft H3 of whichmoves a grounded slider on a resistor RRl in a manner to restore theinput of amplifier 3A2 to Zero. These parts thus form a secondservo-loop whereby the shaft H8 is maintained at all times in an angularposition corresponding to the function K (AR-H).

A second differential D2 receives input from shafts H2 and H8 to formthe sum of R1 and K(AR+J), which sum is Rm in accordance with Equation1a. The function Rm appears at the output of differential D2, shaft H9,to be fed into target range counter GT1 which indicates the targetrange. Shaft H9 also moves sliders on resistors RRIl and RRlo forreasons that will appear when FIGURE 10 is considered, moves a slider ona resistor RRlt) for a reason that will appear when FIG- URE 12 isconsidered, transmits the Rm function to the slider of resistor RR forgeneration of the I function in the manner already described, andoperates a cam GAMd which actuates range change-over switches RG81 andRG82 which change the computer over from short range to long range byaltering the supply point of power to resistor RRltl and its presetbalancing resistors RRltla and RRltlb.

When foot switch FS is reversed to de-energize relay RY, contacts RYltlclose to apply the return signal to servo-amplifier SA2 which causesmotor M3 to center the slider on resistor RRI. Contacts RYI also closeto energize clutch GL1 to connect shaft H3 (clutch GL2 now beingde-energized) to motor M1 for return to zero position under the controlof motor return amplifier MRI which is now connected through contactsRYll to the slider of resistor RRZ. The slider of this AR resistor RRZis thus returned to center position through the zeroing of shaft H3.Contacts RY l cause motor M3 to return the slider of resistor RR tocenter position. These two potentiometers will then remain in thisposition until a new problem is set in and the foot switch FS is againpressed to close relay RY.

The elevation circuit portion (FIGURE Turning now to FIGURE 10, theangle of sight 0 of the antenna ANT is fed to the computer from asynchrotransmitter ST geared to the antenna. This information concerningthe value of 6 is applied to the stator of a control transformer B3 anddevelops an error voltage in its rotor, which voltage is applied as aninput signal to a servo-amplifier 8A3 controlling a motor M4, the outputshaft Hill of which turns the rotor of control transformer B3 to cancelout the error voltage and thus oompletethe servo-loop. Shaft Hltl alsocontrols the slider on resistor RRta as already described in FIGURE 9,feeds to an angle of sight indicator Ill, and is applied as an input toa differential D3.

As already explained, when the system is being used to locate an enemyweapon on the ground, the operator commences by estimating the workingplane WP from a contour map and by setting such estimate of the angleqfi on a working plane handwheel WPH. The shaft Hill of handwheel WPHtransmits motion to a Working plane indicator 12, as well as to a secondinput of differential D3 iii or being the constant. Shaft HlZa feeds I;as in input to another differential D4 where the factor K is derived bysubtracting the function Q from K. See Equation 3 above.

The output of differential D4 (representing K) is applied to shaft H7which, as already indicated, is fed to the slider of resistor RR3 inFIGURE 9. It is also fed to the slider of a resistor RR21 of the azimuthportion of the circuit to be described in connection with FIGURE 11, andto the slider of a resistor RRIZ of a. circuit now to be described.

The factor Q applied to differential D4 by shaft H13 is derived inanother servo-loop formed as a bridge. The inputs are factor Rm atresistor RRll (from FIGURE 9), factor K at resistor RRIZ (which resistorby virtue of additional end turns is made to represent the function K+/z), and AT, the time function, at resistor RR13. (Derivation of factorAT will be explained below.) Servo-amplifier SA4 monitors the output ofthe bridge and controls a motor MS to restore equilibrium by movement ofthe slider of a resistor RR14 forming the fourth arm of the bridge. Theposition of motor M5 is also transmitted by shaft H13 to differentialD4. The resistors RRIZ and RR13 at which factors K'+ /2 and AT areinserted are wound as square law potentiometers. Resistors RRll and RR14are linear. By appropriate adjustment of the constants, the factor Q isobtained at shaft H13 equal to 1 2 2 (K +/2) AT Servo-amplifier 8A4- iscam controlled through contacts CGl from the AT circuit, so that thisservo-loop can only function when AT has been set into the computer.

The AT circuit (FIGURE 10) operates as follows. Motor M6 is acontinuously running synchronous motor and its shaft H14 can beconnected to shaft Hi5 controlling resistor RRlS and cams GAMI and GAMZby means of a clutch GL3. Clutch GL3 is energized through contacts RXIof a latch relay RX controlled by hand switches HS and HS previouslydescribed in connection with FIGURE 7a. The relay RX is of a type whichlatches alternately in and out upon subsequent energizations. Upon beinglatched in by initial actuation of say switch HS as demonstrated inFIGURE 7a, contacts RXl are closed to energize clutch GL3 and transmitthe rotation of the motor M6 to shaft H15 thus beginning to count thefactor AT on timer TM. Indicating lamp LPZ is de-energized at this timeby opening of contacts RX2.

As soon as shaft H15 starts to turn, cam CAMI closes contacts GGl topermit the servo-loop of servo-amplifier 8A4 to operate, and cam GAM2opens its contacts GG2 and closes its contacts GG3. Assuming that handswitch HS or HS is again actuated by the operator before cams GAMI andCAM2 and timer TM have completed one full rotation (as in FIGURE 70),contacts CG GGZ and CO3 remain in their new positions when, upon thesecond closing of switch HS or HS, the relay RX is returned to itsinitial position to de-energize clutch GL3 and leave shaft H15 in theposition is has acquired representing the factor AT. With this ATinformation thus stored in shaft H15, as soon as the foot switch FS isclosed (FIG- URE 7e) a circuit is completed through closed contacts CC3to energize a relay RZ which closes contacts R21 and opens contacts RZ2in the circuit generating the J function (see FIGURE 9). At thesame timeindicating lamp LP1 is lit. In this way the I function can only beapplied when the AT factor has been set in and the foot switch FS is inthe actuated position. However, if the projectile is known to be of thetype having a straight line trajectory such as a self-propelled rocket,it is possible to retain the I function while removing the Q factor fromthe equations. This effect is achieved by closing a rocket switch ROCwhich short circuits the active portion of resistor RR13. The result isan approximately straight line extrapolation, the most appropriate forthis type of projectile.

Another part of the circuit shown in FIGURE is that derived from theshaft H11 of the working plane handwhcel WPI-I to control the positionof a slider on a resistor position of a slider on a resistor RR15. Thispart of the circuit is designed to derive the elevation, which is afunction of the working plane angle and of the range Rm which is appliedby movement of a slider on a resistor RRM (FIGURE 9). This is anotherservo-loop consisting of a servo-an1plifier 3A5 controlling motor M7,the

' output shaft H17 of which adjusts resistor RR17 to rest-ore the inputof amplifier 8A5 to zero while the output from shaft H1? is also fed toanother differential D5. The

known elevation of the radar system is applied at radar elevationhandwheel RLH and is'fed by shaft H16 to appear directly in counter GT3while being added in differential D5 to the difference in elevationbetween the radar system and the target, Rme, as determined by thechosen working plane, to give the elevation of the target in shaft H5and counter CT4.

In operation to locate an enemy weapon, once the operator has a firstreading of the target point, he checks to see if the elevation appearingin counter GT4 agrees with the elevation shown on his map of the targetlocation. If it does not, he turns the working plane handwheel WPH untilthe counter CT4'shoWs the elevation given by The azimuth circuit portion(FIGURE 11) We come now to consideration of the manner of determiningazimuth, for which purpose reference should be made to FIGURE 11.Information concerning the antenna azimuth AA is supplied to thecomputer by an assembly consisting of two synchro-transmitters ST2 and5T3, one-geared directly to antenna azimuth rotation and'the other(Vernier) rotating times per antenna rotationo This synchro informationis applied to the stator of respective main and Vernier controltransformers B4 and B5. Since these rotors are not free to turn, anerror voltage is developed in one or both rotors if they are not aligned.with the actual antenna position." These error voltages are applied toa signal selector SS which receives inputs from the rotors of both themain and vernier transformers B4 and B5. As soon as the input.

from the main control transformer B4 is above a specific voltage levelthen this error signal controls the output :of the signal selector SS.When the error signal from transformer B4 drops below this level, i.e.approaches a null, then any signal being developed by the Verniercontrol transformer B5 becomes the controlling voltage and is passed bythe signal selector SS. The output of signal selector SS passes to aservo-amplifier 8A6, and the servo-loop is completed by a motor Md, theshaft H18 of which is connected to. the rotors of transformers B4 andB5. Motor M8 thus drives the control trans formers to their correctnulls. Should the main control transformer B4 reach its wrong null, i.e.3,200 mils out, then the controlling voltage from the Vernier controltransformer B5 will drive the servo away from this null towards thecorrect one, so that the system will only stabilize itself on thecorrect azimuth setting.

Shaft H18 ofmotor M8 also drives through a differential D6 to appear onan antenna azimuth counter CTS via shaft H19, which also transmits thismotion through a differential D7 to a shaft H20 and a target azimuthcounter GT6. An azimuth orient handwheel AOH controls a shaft H21 alsofeeding into differential D6. In setting up, this handwheel AOH isrotated until the antenna azimuth counter CTS reads the correct bearingof the center of scan of the antenna to a known target. In this way theantenna azimuth is oriented in respect to a compass bearing such asNorth. Shaft H21 is then locked by lock LE1.

Shaft H19 also feeds into a further differential D8 which receives asecond input from the azimuth marker handwheel aMH via shaft H21. Theoutput of differential D8 is recorded in an antenna marker counter CT7.An azimuth potentiometer AZP controlled by shaft H21 controls theposition of the marker spot MS in azimuth on the screen S. In additionto driving differential D8 the azimuth marker handwheel AMH also drivesdifferential D9, 21 i marker counter GT8, and one side of a clutch GL4.

Differential D9 may be considered as a storage device. It functions soas to have an output which indicates the position of the marker spot M5at the first intercept point CML. When the marker spot MS is alignedwith point CML (FIGURE 7e) the foot switch FS is closed to energizeclutch CL4 so that any further motion of the azimuth marker handwheelAMH is applied both directly to differential D9 by shaft H21, and inopposite sign to the same differential through clutch GL4- and shaftH22. Such applications thus subtract from one another. Differential D9effectively stores the azimuth position of the marker spot MS at thepoint CML, namely A1, and passes this information on through shaft H23to differential D10.

Shaft H 22 which only starts to turn after foot switch FS closes clutchCL4 thus provides a measure of AA, the difference in azimuth, and thisfunction is transmitted to a slider on a resistor RR20 of a furtherservo-loop designed to provide the multiple K'AA. The factor K isinserted at the slider of resistor RR21 being obtained from shaft H7 ofthe FIGURE 10 circuit. Resistor RR21 is series connected byco'ntacts'RY7 of relay RY (when energized by foot switch FS). with theslider of resistor RR20. The output which traverses now closed contactsRYS appears at the input of servo-amplifier SA7 to control the positionof motor M9 which in turn.

controls the slider of a resistor RR22 through shaft H25 in a manner torestore the circuit to balance.

The position of shaft HZS-then rcpre'sent'sthe func- 7' tion K'AA whichis applied at differential D10 to be added to factor A1 and hencetransmitted through shaft H24 to be added in differential D7 to theantenna azimuth to provide an output in shaft Hzlltw h ich See Equation2, Am= A1-|-K'AA. This value Am appears in; target azimuth counterCTdand is also fed to a resolver RS represents target azimuth, Am.

shown in more detail in FIGURE 12.

When'foot switch FS is reversed to tie-energize relay RY, contacts RY9close to apply the return signal to to the slider on resistor RRZO. Theslider of this AA- 13 resistor RRZO is thus returned to center positionthrough the zeroing of shaft H22. Contacts RY9 closing causes motor M9to return the slider of resistor RRZZ to center position. These twopotentiometers will then remain in this position until a new problem isset in and the foot switch FS is again pressed to close relay RY.

When echoes appear too close to the edge of the screen S, a centeringdevice incorporated in the circuit portion of FIGURE 11 allows thecentering of the radar scan on the position of these echoes. The amountand direction of rotation of the antenna may be initially determined bythe azimuth marker spot setting or approximated by the operator and seton centering dial CD through shaft H26 by azimuth centering knob ACK.Clutch GL6 and the antenna relays are tie-energized while knob ACK isbeing turned, but operate as soon as torque is removed from thecentering knob. Shaft H26 also controls cams CAM3 and CAM4 whichrespectively operate clockwise antenna rotation switch CWS andcounterclockwise antenna rotation switch CCWS. These switches controlthe antenna ANT so that it will rotate automatically in the directionand the amount set on dial CD.

As a second embodiment of this azimuth centering device (notillustrated), the cams CAMS and CAI/f4 are both attached to shaft H21instead of shaft H20. A suitable clutch arrangement is then providedcoupling the antenna azimuth shaft H18 to the shaft H21. A manual switchprovides excitation for this clutch arrangement as well as for switchesCWS and CCWS which remain mechanically actuated by cams CAM3 and CAM4 tocause the antenna to rotate automatically until the marker spotdisplacement is removed. This thus constitutes a fully automatic azimuthcentering device.

T he information circuit Portion (FIG URE 12 FIGURE 12 shows theinformation portion of the system which is provided to convert thetarget ranges and azimuths to Eastings and Northings. This isaccomplished by obtaining a voltage proportional to target range Rm fromresistor RRIO (see also FIGURE 9) and applying this voltage to thestator RSS of the resolver RS through a booster amplifier BA, while therotor RSR of the resolver RS is rotated in proportion to the targetazimuth Am obtained from the shaft H20 of FIGURE 11. The outputs of thewindings of the resolver rotor RSR are then proportional to Rm sine Amfor Hastings and Rm cosine Am for Northings, such Eastings and Northingsbeing the relative Eastings and Northings of the target in relation tothe radar system. Amplifier ARS and Motor M12 form a servo-loop for theEastings with an output in shaft H30 which drives a slide-r on resistorRRSd until its voltage is exactly equal and opposite to that generatedin the rotor winding of resolver RS to which such slider is connected.Similarly, amplifier AR2 and motor M13 form a servo-loop for theNorthings giving an output in shaft H31 driving the slider of resistorRR31. Resistors RRSG and RR3I are supplied with power from points W, X,Y and Z as indicated from FIGURE 9. A differential D12 driven by shaftH30 has the known radar Eastings applied to it by radar Eastingshandwheel REH and shaft H32. A differential D13 driven by shaft H31 hasthe known radar Northings applied to it by radar Northings handwheel RNHand shaft H34. Absolute radar Eastings appears in counter GT9, while thesum of shafts H39 and H32 appears in shaft H33 as the target Eastings(that is the absolute target Eastings) and is displayed in counter CTNIn a similar manner, absolute radar Northings appears in counter CTlll,while the sum of shafts H31 and H3 appears in shaft H35 as the targetNorthing (that is ab solute target Northings) and is displayed incounter CT12. Shafts H32 and H34 are normally clamped by locks LK2 andLKE.

Manual operation of the system for locating air burst (FIGURES 4 and 10)When the radar system is to be used for calibrating (registering) afriendly gun G, for which purpose it is desired to know the exactlocation of air burst AB, one operator ascends to the roof of thevehicle and the antenna array is oriented so that the angle of sight 6of the lower beam LB is generally in the direction of and slightly abovethe expected point of air burst. The operator at the control panelinside the vehicle observes the two series of echoes which he obtainsfrom the upper and lower beam and feeds the information concerningcenter points CML and CMU, and the time interval AT, into the computerin the manner already explained. The operator on the roof of the vehicleobserves the air burst AB in the telescope TE and aligns the movablegraticule 42 with the burst AB. As indicated in FIG- URE 4, thismovement of the graticule 42 by the knob 43 is transmitted through theshaft H43 to the synchro transmitter 44, whereby the information istransferred electrically to synchro receiver 45 and conveyed by shaftH45 to indicating pointer I4.

The operator now turns the working plane handwheel WP H (FIGURE 10)until the indicating pointer I3 controlled by shaft HIZ is aligned withthe pointer I4. The effect of this movement is to bring the workingplane WP up to the position W? at which it passes through the air burstpoint AB and the working plane angle 5 becomes q It will be clear fromFIGURE 5 that pointer I4 indicates the function A6 and that A0 equals0()=6+', the angle Q5 being assumed positive below the horizontal. Thisis the relationship set up by alignment of the pointers I3 and I4, sinceshaft H45 represents A6 and shaft H12 represents 6-1- 5, and hence 6+This is equivalent to the equati0n=6Ai9.

The solution of the target point coordinates will now emerge from thecomputer to yield the point AB, which is the desired result.

Modification to achieve more automatic operation (FIGURE 10a) FIGURE 10ashows a modified fragment of FIGURE 10. As before, the shaft H10represents the function 0. A further feed from the synchro-transmitterST of the antenna ANT is taken to synchro-diiferential STD, which alsoreceives the function A6 on shaft H43 from the knob 43 of the telescopeTE (FIGURE 4). The electrical output of synchro-ditferential STDrepresents the function 0A9 and is fed to a servo loop comprising asynchro receiver SCR and a servo-amplifier 8A8 which controls a motorM12 the output of which is now connected to the shaft H11 to be fed backinto the synchro receiver SCR in the usual manner of a servo loop. ShaftH11 is still connected to the working plane hand-Wheel WPH, as before,as well as to the indicator 12, the movable tap on resistor RRIS and, asan input, to the differential D3. Shaft Hi1 still represents thefunction or which instead of being inserted manually by the handwheelWPH is now derived from the equation Functions 0 and (,5 are added asbefore in differential D3 and applied through shafts H12 and H12zz tothe differential D4.

This automatic system avoids the need for the operator to align pointersI3 and I4 and feeds the computer automatically as soon as the telescopeoperator aligns the graticule 42 with the burst point AB.

It will be noted that two switches are provided in the power supply tothe servo amplifier 5A8. The first, a spring-loaded, normally-closedswitch S4 is located in the vehicle adjacent the main operators controlpanel. He holds this switch open whenever he wants to turn the workingplane hand wheel WPH manually to move the working plane up or down thetrajectory. Upon release of switch S4, the working plane will bereturned automatically to coincide with the line of sight of graticule42. By thus operating the working plane hand wheel WPH manually theoperator is able to move the computed coordinates up or down thetrajectory, so that, for any other height at which the projectile couldhave been fused to burst, the coordinates are indicated in the maincomputer outputs. The operator may be instructed to set any theoreticalelevation of burst in counter GT4 (FIGURE by moving the working planehandwheel WPH, and the corresponding coordinates of such theoreticalpoint on the computed trajectory appear in counters GT1, GT6, CT10 andC1 12. The upper limit of this computation coincides with the centre ofthe lower beam.

Switch S5 is a further spring-loaded, normally-open switch in the powersupply to amplifier 8A8, switch S5 being located beside the telescopeTE. The telescope operator holds switch S5 closed as soon as he haslined up the graticule 42 with the air burst point AB. Prior to thattime, it is desired to keep the servo loop controlled by servo amplifier5A8 inactive, since the correct value of A0 will not yet have been fedinto the system through shaft H43.

Reconsideration of overall system AMH, the azimuth marker handwheel RHM,the range marker handwheel WPH, the working plane handwheel RLH, theradar elevation handwheel REH, the radar Eastings handwheel RNH, theradar Northings handwheel AOH, the azimuth orient handwheel AT, the timeinterval between observing the projectile in a like position in thelower and upper beams, and

A0, the angle the telescope graticule make with the lower beam centre. V

The information obtained from the antenna comprises 6,'the angle ofsight, and AA, the antenna azimuth.

The principal outputs are in CT 4, the target elevation counter CT6, thetargetv azimuth counter CTI, the target range counter CF10, the targetEastings counter CT12, the target Northings counter Resettingprocedure'wlG URES 9 to 11) After making an observation and computationas above described, the operator resets the computer by depressing thefoot switch F8 for a second time to open its contacts and de-energizerelays RY and RZ. The KAA and K'(AR+J) shafts H25 and H3 are returned tozero by V motors M9 and M3 as described and the AR and AA shafts H3 andH22 are also returned to zero by motors M1 and M10. The operator againpresses his hand switch HS (or. HS) to 're-energize relay RX andre-engage clutch 0L3 so that shaft H again starts to turn. Such motioncontinues until cams CAMI and CAMZ and timer T M each 7 complete one.full revolution. Resistor RR13 0 projectile in flight.

similarly completes one excursion (revolution) to return to its zeroposition. CAM! opens contacts CCl which causes servo-amplifier 3A4 tooperate motor M5 to return the slider of resistor RR14 to zero and thenbecome deenergized. CAMZ opens contacts CC3 and closes contacts CCZ.Closing of contacts CC2 completes a circuit through. contacts RX3 ofrelay RX to re-pulse this relay to return it to and latch it in theposition in which contacts RXl and RX3 are open. This opens clutch CL3to stop shaft H15 with the cams in their zero positions. Now closedcontacts CCZ and RXZ relight lamp LPZ to indicate that resetting hasoccurred.

The system is now in readiness for a new set of values to be fed into itwhen the operator observes another I claim:

l. A radar system comprising (a) means for emitting two closelyvertically superposed, mutually divergent, generally horizontal,etfectively continuous, upper and lower radar beams,

(b) means for displaying echoes returned by a projectile travelling in atrajectory intersecting said upper and lower beams and for determiningrange and azimuth values of such intersections measured from the radarsystem in relation to a known azimuth datum,

(c) a computer,

(d) means for supplying the computer with said determined range andazimuth values, with the value of the angle (0) the lower beam makeswith the horizontal, with the value of the angle (a) between beams andwith the value of the angle (qfi) a working plane makes with thehorizontal,

(e) said computer including means for calculating at least approximatelycoordinates of a target point on said Working plane through which suchtrajectory extends,

V (f) and means for observing the point of burst of a projectile and forsetting the computer for a working plane passing through such observedpoint of burst.

2. A radar system according to claim 1, wherein said last mentionedmeans comprise V (a) means for measuring the angle A0 the lower beammakes with a line joining the radar system and the point of burst,

(b) and means for solving the equation =0A0, and t for setting thecomputer for a working plane having an angle whereby to pass saidworking plane through the observed point of burst.

3. A radarsystem according to claim 1, wherein said computer comprisesmeans for solving equations 4. A radar system according to claim 1,wherein said computer comprises means for solving equations Am.- A 1+KAA where i values respectively,

Tim and Am are the target range and target azimuth V

1. A RADAR SYSTEM COMPRISING (A) MEANS FOR EMITTING TWO CLOSELYVERTICALLY SUPERPOSED, MUTUALLY DIVERGENT, GENERALLY HORIZONTAL,EFFECTIVELY CONTINUOUS, UPPER AND LOWER RADAR BEAMS, (B) MEANS FORDISPLAYING ECHOES RETURNED BY A PROJECTILE TRAVELLING IN A TRAJECTORYINTERSECTING SAID UPPER AND LOWER BEAMS AND FOR DETERMINING RANGE ANDAZIMUTH VALUES OF SUCH INTERSECTIONS MEASURED FROM THE RADAR SYSTEM INRELATION TO A KNOWN AZIMUTH DATUM, (C) A COMPUTER, (D) MEANS FORSUPPLYING THE COMPUTER WITH SAID DETERMINED RANGE AND AZIMUTH VALUES,WITH THE VALUE OF THE ANGLE (O) THE LOWER BEAM MAKES WITH THEHORIZONTAL, WITH THE VALUE OF THE ANGLE (A) BETWEEN BEAMS AND WITH THEVALUE OF THE ANGLE (O) A WORKING PLANE MAKES WITH THE HORIZONTAL, (E)SAID COMPUTER INCLUDING MEANS FOR CALCULATING AT LEAST APPROXIMATELYCOORDINATES OF A TARGET POINT ON SAID WORKING PLANE THROUGH WHCH SUCHTRAJECTORY EXTENDS, (F) MEANS FOR OBSERVING THE POINT OF BURST OF APROJECTILE AND FOR SETTING THE COMPUTER FOR A WORKING PLANE PASSINGTHROUGH SUCH OBSERVED POINT OF BURST.